The present invention pertains to non-volatile semiconductor magnetoresistive random access memory (MRAM) chips and more particularly is concerned with double-decker MRAM cells where each one of the cells comprises two stacks of magnetic tunnel junctions.
An MRAM cell (also referred to as a tunneling magnetoresistive or TMR-device) includes a structure having ferromagnetic layers respectively exhibiting a magnetic moment vector separated by a non-magnetic layer (or tunneling barrier) and arranged into a magnetic tunnel junction (MTJ). In contrast to present day's non-volatile DRAM memory technology, digital information is not stored by charge but rather is represented in the MRAM cell as directions of magnetic moment vectors (magnetization) in the ferromagnetic layers. More specifically, the magnetic moment vector of one ferromagnetic layer is magnetically fixed (or pinned), while the magnetic moment vector of the other ferromagnetic layer is free to be switched between the two preferred directions in the magnetization easy axis, which typically is arranged to be aligned with the fixed magnetization of the reference layer. Hence, a memory state of an MRAM cell is maintained by the direction of the magnetization of the free layer with respect to the direction of the magnetization of the reference layer. Depending upon the two different magnetic states of the free layer, the MRAM cell exhibits two different resistance values in response to a voltage applied across the magnetic tunneling junction barrier. Accordingly, the particular resistance of the TMR-device reflects the magnetization state of the free layer. In this way, the resistance is low when the magnetization of the free layer is parallel to the magnetization of the reference layer, and high when magnetizations are antiparallel. Hence, a detection of changes in resistance allows to provide information stored in the MRAM cell.
In order to switch MRAM cells, magnetic fields that are coupled to the freely switchable magnetization of the magnetic free layer are applied, which typically are generated by supplying currents to current lines, for example, write bit and write word lines, usually crossing at right angles with an MRAM cell being positioned in an intermediate position therebetween and at an intersection thereof.
Recently, a new concept of MRAM cells (“toggle cells”) has been proposed, wherein the free layer is designed to be a free magnetic region including a number of ferromagnetic layers that are antiferromagnetically coupled, where the number of antiferromagnetically coupled ferromagnetic layers may be appropriately chosen to increase the effective magnetic switching volume of the MRAM device. See, for instance, U.S. Pat. No. 6,531,723 B1 to Engel et al., the disclosure of which is incorporated herein by reference.
For switching such magnetoresistive memory cells having a free magnetic region including antiferromagnetically coupled ferromagnetic layers, another switching mechanism, the so-called “adiabatic rotational switching”, which is well-known to the skilled persons, is envisaged. The adiabatic rotational switching mechanism is, for example, disclosed in U.S. Pat. No. 6,545,906 B1 to Savtchenko et al., the disclosure of which is incorporated herein by reference. More specifically, adiabatic rotational switching relies on the “spin-flop” phenomenon, which lowers the total magnetic energy in an applied magnetic field by rotating the magnetic moment vectors of the magnetic free region ferromagnetic layers.
Now reference is made to FIG. 1A, where a typical stability diagram for an adiabatic rotation switchable MRAM cell is illustrated, the abscisse of which represents the bit line magnetic field HBL, while the ordinate represents the word line magnetic field HWL, which respectively arrive at the MRAM cell for its switching. Using the spin-flop phenomenon in an MRAM cell having antiferromagnetically coupled magnetic moment vectors M1 and M2 of the free magnetic region ferromagnetic layers inclined at a 45° angle to the word and bit lines, respectively, a timed switching pulse sequence of applied magnetic fields in a typical “toggling write” mode is described as follows.
At a time to neither a word line current nor a bit line current are applied resulting in a zero magnetic field H0 of both HBL and HWL. At a time t1, the word line current is increased to result in magnetic field H1 and magnetic moment vectors M1 and M2 begin to rotate either clockwise or counter-clockwise, depending on the direction of the word line current, to align themselves nominally orthogonal to the field direction. At a time t2, the bit line current is switched on. The bit line current is chosen to flow in a certain direction so that both magnetic moment vectors M1 and M2 are further rotated in the same clockwise or counter-clockwise direction as the rotation caused by the word line magnetic field. At this time t2, both the word and bit line currents are on, resulting in magnetic field H2 with magnetic moment vectors M1 and M2 being nominally orthogonal to the net magnetic field direction, which is 45° with respect to the current lines.
At a time t3, the word line current is switched off, resulting in magnetic field H3, so that magnetic moment vectors M1 and M2 are being rotated only by the bit line magnetic field. At this point of time, magnetic moment vectors M1 and M2 have generally been rotated past their hard axis instability points. Finally, at a time t4, the bit line current is switched off, again resulting in zero magnetic field H0, and magnetic moment vectors M1 and M2 will align along the preferred anisotropy axis (easy axis) in a 180° angle rotated state as compared to the initial state. Accordingly, with regard to the magnetic moment vector of the reference layer, the MRAM cell has been switched from its parallel state into its anti-parallel state, or vice versa, depending on the state switching (“toggling”) starts off with.
In order to successfully switch the MRAM cell, it is essential that the magnetic field sequence applied thereon results in a magnetic field path crossing a diagonal line being a straight connection between a minimum switching field HSF (“toggling point”) for reversal of the free magnetization and another critical magnetic field value HSAT (“saturation point”), at which both magnetic moment vectors M1 and M2 of antiferromagnetically coupled ferromagnetic layers of the free magnetic region are forced to align with the applied external magnetic field in a parallel configuration.
Usually, the first and third quadrant of the HBL-HWL-plane are used for switching the cell. Apparently, as can be seen from FIG. 1A, no magnetic fields are applied in the second and fourth quadrant leaving room to operate another (second) magnetic tunnel junction in the same memory cell, the reference layer magnetization is rotated by 90 degrees relative to the first one.
Reference is now made to FIG. 1B. Assuming a second magnetic tunnel junction similar to above (first) magnetic tunnel junction except that it is rotated by 90°, a timed switching pulse sequence of applied magnetic fields in the second quadrant is typically as follows: a time t0 neither a word line current nor a bit line current are applied resulting in a zero magnetic field H0 of both HBL and HWL. At a time t1, the word line current being reversed to the previous case is increased to result in magnetic field H1 and magnetic moment vectors M1 and M2 of the second MTJ begin to rotate either clockwise or counter-clockwise, depending on the direction of the word line current, to align themselves nominally orthogonal to the field direction. At a time t2, the bit line current is switched on. The bit line current is chosen to flow in a certain direction so that both magnetic moment vectors M1 and M2 are further rotated in the same clockwise or counter-clockwise direction as the rotation caused by the word line magnetic field. At this time t2, both the word and bit line currents are on, resulting in magnetic field H2 with magnetic moment vectors M1 and M2 being nominally orthogonal to the net magnetic field direction, which is 45° with respect to the current lines. At a time t3, the word line current is switched off, resulting in magnetic field H3, so that magnetic moment vectors M1 and M2 are being rotated only by the bit line magnetic field. At this point of time, magnetic moment vectors M1 and M2 have generally been rotated past their hard axis instability points. Finally, at a time t4, the bit line current is switched off, again resulting in zero magnetic field H0, and magnetic moment vectors M1 and M2 will align along the preferred anisotropy axis (easy axis) in a 180° angle rotated state as compared to the initial state. Accordingly, with regard to the magnetic moment vector of the reference layer, the second MTJ of the MRAM cell has been switched from its parallel state into its anti-parallel state, or vice versa, depending on the state switching starts off with. As with the first cell, magnetic field sequence applied on the second MTJ crosses a diagonal line being a straight connection between a minimum switching field HSF for reversal of the free magnetization and another critical magnetic field value HSAT, at which both magnetic moment vectors M1 and M2 of antiferromagnetically coupled ferromagnetic layers of the free magnetic region are forced to align with the applied external magnetic field in a parallel configuration.
As above described, in order to successfully switch two different MTJs in a single memory cell, it is necessary that the free layer magnetizations are inclined at an angle of 90°. Such situation is illustrated in FIG. 2, where a stacked structure 1 of two magnetic tunnel junctions (MTJs) of a memory cell is positioned in between bit and word lines at an intersection thereof and having free and reference layer magnetizations 2, 3 exhibiting a 90° angle in between. (FIG. 2 illustrates different cases of orientations of the two free layer magnetizations, each one having a 90° angle in between.) A possible realization of two different MTJs in a single cell is the so-called “double-decker MRAM cell”-concept having a stacked structure of two MTJs. Using such a double-decker MRAM cell allows for storing two bits of information in a single memory cell. Thus, half the effective cell size per MTJ as compared to the convenient case having only one MTJ per memory cell can be achieved. However, in such double-decker MRAM cell, reference layer magnetizations have to be inclined in an angle of 90° in order to selectively read the MTJs for which reason the pinning layers for pinning of the reference layers in the state of the art necessarily have to be made of different antiferromagnetic materials having sufficiently different setting (Neel) temperatures. Accordingly, optimizing the antiferromagnetic materials such that they have as high a difference in setting temperatures as possible while meeting other requirements like pinning strength, thermal stability etc. is a big challenge and often results in a rather dissatisfying trade-off of desired characteristics. Accordingly, there is a need for the present invention.